1. Field of the Invention
The present invention relates to a nitride semiconductor light-emitting device, and a method for manufacturing the same.
2. Description of the Related Art
An example of a related art nitride semiconductor may include a GaN-based nitride semiconductor. The GaN-based nitride semiconductor is being applied to a variety of application fields such as an optical device including blue/green light emitting diodes (LEDs), and a fast-switching high-output device including a metal-semiconductor field effect transistor (MESFET) and a high electron mobility transistor (HEMT). Particularly, the blue/green LEDs have already been mass-processed, and the worldwide sales thereof are exponentially increasing.
Particularly, in the field of light emitting devices such as LEDs and semiconductor laser diodes among the application fields of the GaN-based nitride semiconductor, a semiconductor light emitting device that includes a crystal layer made by doping a Ga location of a GaN-based nitride semiconductor with the group 2 element such as magnesium and zinc is receiving much attention as a blue light emitting device.
As illustrated in FIG. 1, an example of the GaN-based nitride semiconductor may include a light-emitting device having a multiple quantum well structure. The light-emitting device is grown on a substrate 1 formed mainly of sapphire or SiC. For example, a polycrystalline thin film of AlyGa1-yN is grown as a buffer layer 2 on the substrate 1 of sapphire or SiC at a low growth temperature. Then, a GaN underlayer 3 is sequentially stacked on the buffer layer 2 at a high temperature. An active layer 4 for light emission is placed on the GaN underlayer 3. A magnesium (Mg)-doped AlGaN electron barrier layer 5, a Mg-doped InGaN layer 6, and a Mg-doped GaN layer 7 that are converted into a p-type by a thermal annealing process are sequentially stacked on the active layer 4.
An insulating layer is formed on the Mg-doped GaN layer 7 and the GaN underlayer 3, and a p-type electrode 9 and an n-type electrode 10 are formed corresponding to the Mg-doped GaN layer 7 and the GaN layer 3, respectively, thereby forming a light emitting device.
Such a related art light emitting device has the following problems. First, in the nitride semiconductor light-emitting device, lattice mismatch exists between the substrate and the GaN, and thus many crystal defects (surface defects, point defects, line defects) occur in an n-type nitride layer or a p-type nitride layer which has undergone crystal growth. Hence, it becomes difficult to achieve good quality of a crystal layer.
Also, at the time of Mg doping for forming a p-type contact layer, Mg is combined with H of an ammonia gas, thereby forming a Mg—H combination having an electrical insulating property. Thus, even if a large amount of Mg is doped, it is difficult to achieve high hole-concentration in a p-type GaN.
In general, it has been known that quantum dots formed in InGaN and GaN epilayers used as an active layer take prominent part in increasing attention on the nitride semiconductor material as a high-output optical device despite its disadvantages of dislocations, defects and an electromagnetic field in a crystal. Such quantum dots perform strong lateral confinement or localization on carriers (electrons and holes) to remarkably reduce an influence of the dislocation or the electromagnetic field.
Specifically, electrons of a conduction band and holes of a valence band in an active layer having a quantum well structure are trapped in the quantum dots, increasing the density states of the electrons and the holes in the quantum dots. Thus, light-emission recombination efficiency of the electrons and the holes effectively increases. Also, a refractive index of the quantum dot is greater than a refractive index of a semiconductor material surrounding the quantum dot. For this reason, photons generated at the quantum dots are spatially trapped near to the quantum dots. Such an active layer structure of the light emitting device simultaneously confines both carriers and photons in the center of an optical waveguide, so that a threshold current of the light emitting device can be reduced approximately tens of times, and temperature stability can be improved to an extent that allows consecutive operation of the light emitting device at a room temperature.
Accordingly, in order to improve light emission efficiency of the nitride semiconductor light-emitting device, it is most important to develop a technology for controlling quantum dots of the active layer.